1. Field of the Invention
The present invention relates to an x-ray mask blank, an x-ray mask in which a silicon carbide film is used as an x-ray membrane, a method of manufacturing the same, and a method of polishing the silicon carbide film.
2. Description of the Related Art
In the semiconductor industry, as a well known technique for forming an integrated circuit, constituted of a fine pattern on a silicon substrate or the like, uses a photolithography method for transferring the fine pattern, with visible light and ultraviolet light used as an exposing electromagnetic wave. However, a recent advance in a semiconductor technique greatly promotes a high integration semiconductor device, such as VLSI, and this requires that the fine pattern be transferred with high accuracy beyond the transfer limit (a principled limit due to a wavelength) of visible light and ultraviolet light, for use in the conventional photolithography method. To transfer such a fine pattern, an x-ray lithography method uses an x-ray having a wavelength that is shorter than the wavelength of visible light and ultraviolet light.
FIG. 1 is a cross sectional view showing a structure of an x-ray mask for use in the x-ray lithography. FIG. 2 is a cross sectional view showing an example of an x-ray mask blank structure as an intermediate product obtained in an intermediate process during the manufacturing of the x-ray mask.
As shown in FIG. 1, an x-ray mask 1 comprises an x-ray membrane 12, for transmitting the x-ray, and an x-ray absorbing film pattern 13a formed on the x-ray membrane 12. The x-ray membrane 12 is supported by a silicon frame body 11a which is formed by removing the inner portion so that only the periphery of the silicon substrate remains. When the x-ray mask 1 is manufactured, the x-ray mask blank is manufactured in the intermediate process as an intermediate product. The x-ray mask blank is further processed, and the x-ray mask is obtained. In this industry, although, of course, the x-ray mask, which is a finished product, is to be dealt in, and the x-ray mask blank, which is the intermediate product, is also often to be independently dealt in.
As shown in FIG. 2, an x-ray mask blank 2 comprises the x-ray membrane 12 formed on a silicon substrate 11 and an x-ray absorbing film 13 formed on the x-ray membrane.
Silicon nitride, silicon carbide, diamond or the like may generally be used as the x-ray membrane 12. An amorphous material, including tantalum (Ta), having an excellent resistance to x-ray radiation is often used as the x-ray absorbing film 13.
The following method may be used for manufacturing the x-ray mask 1 from the x-ray mask blank 2. A resist film on which a desired pattern is formed is arranged on the x-ray mask blank 2, shown in FIG. 2. This pattern is then used as a mask, so as to perform a dry etching so that the x-ray absorbing film pattern is formed. After that, a center area, formed on a rear surface and to be a window area of the x-ray membrane 12, is removed by a reactive ion etching (RIE) using 4-fluorocarbon (CF.sub.4) as etching gas. The remaining film (12a: see FIG. 1) is then used as the mask so as to etch the silicon by an etching liquid constituted of a mixed liquid of fluoric acid and nitric acid, whereby the x-ray mask 1 (see FIG. 1) is obtained. In this case, an electron beam (EB) resist is generally used as the resist, and the pattern is formed by means of an EB lithography.
The x-ray membrane 12 requires a high transmittance to an x-ray, a high Young's modulus of elasticity, a proper tensile stress, a resistance to x-ray radiation, high transmittance within a visible light range, and the like. These characteristics will be described below. The transmittance to the x-ray is required during exposure. The higher the transmittance, the shorter the exposure time can. This is effective for improving throughput. The Young's modulus of elasticity influences the strength of the film and the deformation of an absorber pattern. The higher the Young's modulus of elasticity, the higher the film strength becomes. This is effective for suppressing misalignment. A proper tensile stress is needed so the film is self-supported. Resistance to x-ray radiation is required so that no damage results from x-ray radiation, because the x-ray membrane is irradiated with the x-ray during exposure. With regard to the transmittance within the visible light range, because an alignment of the mask attached to an x-ray stepper and a wafer is accomplished by the use of a light source within the visible light range, the high transmittance to an alignment light source is needed in order to achieve a highly accurate alignment. Furthermore, a smooth film surface is required. Surface smoothness is needed so that a highly accurate pattern may be formed on the absorber.
In order to satisfy these requirements, various materials and manufacturing methods have been studied. Since silicon carbide has the highest Young's modulus of elasticity and causes no damage due to the x-ray in the silicon nitride, the silicon carbide (SiC) and the diamond which have been heretofore used as the x-ray membrane, it may safely be said that the silicon carbide is the most promising material. However, because the SiC film has a polycrystalline structure, the SiC film has a film surface which is rougher than 6 nm (Ra: a center-line average roughness) due to its crystalline structure. For smoothing the surface of this SiC film, an etch back method and a mechanical polishing method are carried out after film formation. The etch back method is the technique in which the rough SiC film is coated with the resist. The thus obtained smooth resist surface is transferred onto the SiC film by dry etching. The mechanical polishing is the method in which a hard grain, such as the diamond and alumina, is used as an abrasive material so as to physically grind an unevenness on the surface of the SiC film. For example, according to Japanese Patent Publication No. 7-75219, the surface roughness of 20 nm or less is obtained by the etch back and mechanical polishing. Although a definition of surface roughness is not clear in this publication, such roughness is expected to be a maximum height (Rmax) and corresponds to about 2 nm or less in terms of Ra.
Recently, due to advances in photolithography an introduction of the x-ray lithography has been performed later. At present, the introduction from a generation of 1 G bit-DRAM (design rule: 0.18 .mu.m) is anticipated. Even if the x-ray lithography is introduced from 1 G, the x-ray lithography is characterized in that it can be used through a plurality of generations up to 4 G, 16 G and 64 G. Assuming that x-ray lithography is used for 64 G, position precision required for the x-ray mask becomes severer, and the position precision is required to be as high as 10 nm. Furthermore, the mask pattern is required to have no defect regardless of pattern size. Although the pattern defect can be corrected by a defect correcting unit, the number of practically correctable defects is limited to about 10 or less on the mask surface. A factor causing pattern defect is the defect of the x-ray absorbing film. More specifically, an important factor is the defect of the x-ray membrane. That is, if the defect (a contaminant or the like) is caused on the membrane, the defect is also inherited on the absorbing film formed on the membrane. Moreover, this faulty absorbing film causes the pattern defect after a mask processing. Therefore, it is necessary to exactly check defects on the x-ray membrane and to process so that no defect or the least defect may be on the membrane. A minimum defect size affecting the pattern defect corresponds to the width of the minimum pattern line. Therefore, for the x-ray mask, it is necessary to exactly check the defect size to about 0.2 .mu.m.
To check for defects, a laser light is used in a method for detecting the light scattering from any surface defect. For example, by Surf scan 6220 (KLA Tencor), a minimum sensitivity of 0.09 .mu.m can be realized on a silicon wafer. However, in such a surface defect checking unit using a laser light, the level of sensitivity for detecting the defect is influenced by surface roughness. Therefore, if the surface is rough, the roughness causes the light to scatter. Thus, fine defects cannot be recognized (distinguished). The inventors have repeated experiments as to the surface roughness and the size of the defect to be detected. As a result, in order to exactly evaluate a defect of 0.2 .mu.m, it is confirmed that surface roughness must be 1.0 nm (Ra) or less. However, when the etch back method is used, although a roughness of 2 nm or less is obtained, the smooth surface of 1 nm or less can be scarcely obtained. On the other hand, in the mechanical polishing method, since the SiC film is a very hard film, the film can be scarcely ground by a soft abrasive particle such as silica, in general. By the use of a hard abrasive material such as a diamond, a smooth surface of 1 nm or less can be obtained. However, it has been confirmed that the use of a diamond particle easily causes a scratch on the surface. It is confirmed that such a scratch of a certain size or more causes not only a reduction of the film strength but also breakage (defect) of the fine pattern.